1. Field of the Invention
This invention relates to thermal circuit breakers.
2. Description of the Related Art
A number of circuit breakers are known in which a bimetallic element responds to an overcurrent through the breaker by physical deformation so as to trip the breaker, interrupting the current. See, for example, U.S. Pat. No. 4,510,479, which is in the name of the present inventor and which is commonly assigned with the present application. The breaker shown in that patent has a pivoted contact arm carrying one of the contacts of the breaker. A bimetallic strip carries the other contact of the breaker. When an overcurrent passes through the bimetallic strip, it deforms urging the contact arm to move against the bias of an overcenter spring. When the bimetallic element forces the pivoted contact member past the overcenter point, the breaker snaps open, breaking the circuit.
The breaker shown in the prior patent referred to above involves a compromise relating to the spring pressure urging the contact on the moving contact arm against the contact carried by the bimetallic member. That is, since the bimetallic member must move the contact arm against the over center spring bias in order to trip the breaker, the spring force must be less than the force developed by the bimetallic member in response to an overcurrent. Where the breaker is of relatively low current rating, such that a relatively low current is required to deform the bimetallic element and trip the breaker, the spring pressure must be reduced correspondingly. In some cases, the force urging the moving contact on the contact arm against the contact carried by the bimetallic element was occasionally insufficient to provide good electric contact therebetween. Hence, a voltage drop across the contacts of the breaker was noted. For similar reasons, the breaker shown in that patent occasionally exhibited circuit interruption due to vibration; that is, vibration of the breaker would cause the pivoted contact arm to bounce away from the contact on the bimetallic member, even against the bias of the overcenter spring.
Other known thermal breaker designs have involved the latching of a spring biased contact arm carrying a movable contact by a bimetallic element, in which the end of the bimetallic element is received by a ledge or other recess on a surface of the moving contact arm. When the bimetallic element flexes due to heating (which can be due to passage of an overcurrent therethrough or due to heating of the ambient air within the breaker housing, caused by current passing through a separate heating element) the edge of the bimetallic element is pulled out of engagement with the retaining ledge, releasing the movable contact. The edge of such bimetallic elements has typically been a sharp stamped edge, often having a burr, which is received by the retaining ledge. The friction between the burr on the bimetallic element and the ledge can be quite high, such that the breaker is prevented from releasing properly. This can lead to wide variations in breaker current ratings and to improper operation.
Examples of breakers in which an edge of the bimetallic element engages a retaining ledge include Fleming U.S. Pat. No. 2,504,513, Von Hoorn U.S. Pat. No. 2,150,013, and Landmeier U.S. Pat. No. 2,146,266. Landmeier also suggests that such a breaker structure can be provided in a "tripfree" configuration such that an individual can not override the thermal tripping function, e.g., by holding the breaker actuating handle in the "ON" position.
U.S. Pat. No. 4,338,586 to Scanlon shows a circuit breaker in which a pivoted latch lever has a detent for restraining movement of a slidable latch. When a bimetallic element is heated by an overcurrent therethrough, it engages the latch lever and pivots it away from the slidable latch, moving the detent out of engagement with the latch. The latch then moves, allowing a movable contact arm to pivot in response to spring bias provided by the resilient nature of the movable contact arm. The contacts then open.
In the Scanlon design, the force exerted by flexing of the bimetallic element does not have to directly overcome the bias of the movable contact arm, inasmuch as the bimetallic element controls the motion of a latch. However, the bimetallic element does not itself latch a locking member. Instead, the bimetallic element contacts the latch lever, which in turn controls the slidable latch. A multiplicity of parts is thus provided, such that it would appear very difficult to provide the Scanlon breaker in a very small package. The large number of parts in the Scanlon design would also appear to render it relatively expensive to produce.
Furthermore, the Scanlon breaker does not appear to provide ambient temperature compensation; that is, it does not in any way distinguish between deformation of the bimetallic element due to variations in ambient temperature and due to passage of an overcurrent therethrough. Accordingly, the trip point of the Scanlon breaker would naturally vary with variations in ambient temperature.